I have a machine that takes in power using a 5-pole plug (3-phase 480 + 1 Neutral + 1 Earth Ground). The machine powers a 3-phase motor as well as using one of the phases along with the neutral that goes to an auto-transformer that steps the voltage down to 120VAC to power the DC Power supplies for the control circuitry.

My problem right now is that my 480V services only has 4 poles (3-phase 480 + 1 Earth Ground). Can I convert my machine to a 4-pole design. From my research I think i can use 2-phases to the step-down transformer (i'll just have to adjust the transformer to get the 120VAC). Is this correct?

The only other potential problem is i'm using a ABB Residual Current breaker to detect faults that is 4-pole (3-phases and a 1-neutral). Can i still use this breaker with the neutral switched for a ground conductor?

1/2/2015 10:10:08 AM

You're an electrical engineer, right?

1/2/2015 12:21:47 PM

I've never designed 3-phase systems that were more complicated than just a motor downstream from the motor starter and circuit protection.

1/2/2015 1:49:50 PM

As it seems others are slow to answer, I suspect you should not use 2-phases to get 1-phase power to power your control circuitry. Math suggests attempting to do so will increase the voltage to your 1-phase step down transformer about 73%. Maybe you can tweak everything to still work fine, but you'll need to keep the higher resultant voltages in mind.

I wouldn't know about the breaker.

1/4/2015 11:59:14 PM

what do you mean your 480V services only have 4-poles? as in the physical branch circuit at your machine was only pulled with 3-phase+ground and is equipped with a NEMA #15-xx receptacle?



trying to jerry rig something like this is a great way to put yourself and others in danger. if you don't have the correct circuit available why don't you put a work order in and have the maintenance team pull a new circuit with correct phasing and amperage to the machine?

1/5/2015 10:23:39 AM

I'm so proud of jeepin...

Seriously though... pull new cable... make sure the circuit is designed for what you are using it for and has the proper breaker on it.

This will require a permit.....

1/5/2015 4:43:11 PM

First off, don't toy around with this stuff if you don't know what you're doing. I'm only providing the below information for discussion. You left out way too much detail about what you're doing for any real assistance.

you can pull single phase power from two legs of a delta connection. the current between all three phases will still be balanced, so that shouldn't trip a Residual Current Device. The ground should not be attached to the breaker unless you want the breaker to not trip under leakage current situations (AKA getting electrocuted). How you plan on "adjusting" an autotransformer is a mystery to me; hopefully you mean obtaining a different transformer than the one you have now.

A 4 pole service denotes a wye connection and a 3 pole service denotes a delta connection. You don't count the ground when denoting poles.

Why do you only have a delta service now? if your current source is only fed from a delta service, then pulling new wire isn't an option. You could possible get a 480V:480V delta/wye transformer and create a local neutral to feed what you currently have.

1/7/2015 1:13:15 AM

maybe HUR proceeded on his own and came to an untimely demise.

1/7/2015 9:59:05 AM

HUR exposed

1/7/2015 3:37:04 PM